Acid gases are present as impurities in numerous industrial fluids, i.e., liquid and gas streams. These acid gases include hydrogen halides such as HCl, HF, HBr, HI and mixtures thereof. For example, one of the key processes in refining petroleum is catalytic reforming. In the catalytic reforming process, a light petroleum distillate or naphtha range material is passed over a noble metal catalyst to produce a high octane product. Hydrogen is a by-product of the catalytic reforming process, and a portion of the byproduct hydrogen is recycled to the reaction zone to maintain catalyst stability. Typically, the noble metal reforming catalyst is promoted with chloride which, in the presence of hydrogen, results in the production of a small amount of hydrogen chloride. Thus, the net byproduct hydrogen withdrawn from the catalytic reforming process generally contains a small amount of hydrogen chloride. Similarly, in a process for the dehydrogenation of light isoparaffins to produce iso-olefins, the promoting of the noble metal catalyst with chloride will produce a net hydrogen stream containing small amounts of HCl. The net hydrogen produced in the catalytic reforming process and the dehydrogenation process is generally used in sensitive downstream catalytic processes. In addition, there are other hydrocarbon and chemical processes in which small amounts of HCl are generated and carried away in gas or liquid streams. Even small amounts of gaseous HCl present in the net hydrogen can seriously interfere with the operation of downstream processes which use the hydrogen and can cause corrosion problems in the equipment such as pipes, valves, and compressors which convey hydrogen. Generally, HCl in gas or liquid hydrocarbon streams must be removed from such streams to prevent unwanted catalytic reactions and corrosion to process equipment. Furthermore, HCl is considered a hazardous material and releasing the HCl to the environment must be avoided.
Currently, activated alumina is the most widely used sorbent in the petroleum refining and chemical industries. Activated alumina is employed as a scavenger for the removal of small quantities of HCl from fluid streams. Significant developments to improve the performance of alumina to remove HCl from hydrocarbon streams are disclosed in U.S. Pat. No. 4,639,259 and U.S. Pat. No. 4,762,537 which relate to the use of alumina-based sorbents for removing HCl from gas streams. U.S. Pat. No. 5,505,926 and U.S. Pat. No. 5,316,998 disclose a promoted alumina sorbent for removing HCl from liquid streams by incorporating an alkali metal oxide such as sodium in excess of 5% by weight on to an activated alumina base. It is also known that alumina can be promoted to adsorb more HCl by impregnating the alumina with sodium carbonate or sodium hydroxide or calcium hydroxide. U.S. Pat. No. 4,639,259 discloses the use of calcium acetate to improve the dispersion of the calcium oxide on the alumina to achieve higher sorption capacity. The use of promoted alumina compared to other alumina-based sorbents can extend the length of time a fixed amount of sorbent will adsorb HCl. By increasing the content of promoters such as sodium carbonate or sodium hydroxide, the HCl sorption capacity of the scavenger can be increased. However, the addition of promoters to alumina to improve the capacity of the sorbent for HCL appears to have a point of diminishing returns. Despite the type and amount of promoter incorporated into the alumina-based and promoted alumina materials, commercial experience shows that alumina-based and promoted alumina sorbents have a relatively low capacity for the sorption of HCl, often limited to levels less than 10 to 16 wt-% HCl.
Existing sorption processes for removing HCl from hydrocarbon-containing streams typically involve passing the hydrocarbon-containing fluid stream over the sorbent, which is disposed in a fixed bed. Conventionally, these fixed beds contain alumina-based sorbents wherein sodium or calcium is doped or coated on the alumina. Typically, the alumina-based and promoted alumina materials are formed into nodules or spheres. As the alumina-based sorbents pick up HCl, the sodium or calcium promoter, as well as the aluminum, reacts with HCl to form chloride salts. Because HCl molecules are able to form hydrogen bonds with chloride ions, a limited amount of HCl can become physically adsorbed on the surface of the salt molecules. However, the alumina sorbent in this service is known to have the undesirable property of converting certain hydrocarbons in the streams into a substance often called “green oil” which often collects in the fixed sorbent bed. Typically, these green oils are green or red in color. They generally contain chlorinated C6 to C18 hydrocarbons and are believed to be oligomers of light olefinic hydrocarbons. The presence of green oils in the fixed sorbent bed fouls the sorbent bed and results in the premature failure of the sorbent. When this fouling occurs, often costly measures are required to remove the spent sorbent from the bed. Furthermore, the chloride content of the green oils on the spent sorbent makes disposal of the spent sorbent an environmental problem. While the exact mechanism of green oil formation is unknown, it is believed that green oils are formed by catalytic reaction of aluminum chloride or HCl with the hydrocarbon. Green oil formation remains an unresolved industry problem during the removal of HCl from hydrocarbon streams.
When unsaturated hydrocarbons such as butadiene or other olefinic compounds are present in a hydrocarbon-containing stream, these compounds can be polymerized on acidic surfaces. Alumina based sorbents and promoted alumina sorbents, once they adsorb HCl, become acidic during the sorption process, and thus, acquire catalytic activity for the polymerization of the reactive hydrocarbons in the stream. When green oils are produced during the HCl sorption process, the spent sorbent represents a costly disposal problem. The formation of these polymers fouls the adsorbers, shortens sorbent life, and creates a problem for the disposal of the solid adsorbents now containing chlorinated hydrocarbons. Since an HCl sorbent is not regenerable, the treatment of streams with even moderate to high HCl content, such as an HCl sorbent with a capacity of 10 to 16 wt-%, requires the fixed bed of sorbent to be changed frequently and imposes a downtime on the upstream process. Because the change of sorbent beds containing polymerized hydrocarbons requires costly measures to dig the sorbent out of the sorbent bed, the loss of production time and the maintenance costs are especially significant. The polymerization or acidic reactivity of the Cl loaded adsorbents must be reduced to avoid these problems.
There are many compounds that are reactive to acid gases such as hydrogen halides which can be employed as a scavenger sorbent to remove trace amounts of acid gases from fluid streams. However, for a compound to function in a fluid stream from a process plant where hydrocarbons are present, the material must have good acid gas sorption capacity, have sufficient physical strength, and be catalytically inert in the presence of reactive hydrocarbons. Acid gases are present as contaminants in various industrial gas and liquid streams. The catalytic reforming process, which is widely employed for producing high quality gasoline components, is one of the major sources of HCl contaminated fluids. The catalysts used in the catalytic reforming are commonly promoted with chloride compounds. These compounds slowly come off the catalyst in the course of use, thereby contaminating some product streams with HCl and other contaminants. It is often necessary to remove these types of contaminants to prevent corrosion and fouling problems downstream. The present invention provides a material for removal of such contaminants.
It has been generally recognized that the efficiency of a chloride scavenger depends not only upon the content of the active metal incorporated into the material but also upon at least two more factors: the accessibility of the active component and the reactivity of the scavenger towards the main stream. In addition, the adsorbents should have sufficient mechanical strength to withstand the loading-unloading operations and other disturbances while in service.
Initially, activated (non-doped) aluminas were used for industrial removal of HCl. The low capacity of the activated aluminas and the high reactivity of the spent material resulting in the formation of “green oil” motivated the search for improved scavengers. Incorporation of alkali or alkaline earth elements into alumina carriers was then found to significantly improve the performance of the chloride scavengers. For examples of such use, see U.S. Pat. No. 3,557,025; U.S. Pat. No. 3,943,226; U.S. Pat. No. 4,639,259; U.S. Pat. No. 5,505,926; and U.S. Pat. No. 5,935,894.
U.S. Pat. No. 3,557,025 teaches the preparation of an alkalized alumina for removal of SO2. This alkalized alumina which was made from a combination of alumina with a carbonate required the addition of alumina and sodium bicarbonate to a water suspension, followed by autoclaving for a period of hours.
Another early process for preparation of an alumina/carbonate product is described in U.S. Pat. No. 3,518,064 in which is disclosed a reaction of an alkali metal bicarbonate or ammonium bicarbonate and aluminum hydroxide that are heated together in the dry state. The product was to be used as a buffering composition especially in treatment of gastric hyperacidity.
More recent efforts to develop a useful adsorbent for removal of HCL from hydrocarbon streams include U.S. Pat. No. 6,060,033 and U.S. Pat. No. 6,200,544. In the '033 patent, Cheng taught that the preloading of water on a sodium promoted alumina adsorbent increased the HCl sorption capacity and decreased the catalytic reactivity of the adsorbent. Recently, in U.S. Pat. No. 6,200,544, Blachman taught the use of an adsorbent for removing HCl from fluid streams comprising activated alumina impregnated with alkali oxide and promoted with phosphate or organic amine or a mixture thereof.
In an attempt to increase the adsorbent performance, U.S. Pat. No. 5,897,845 disclosed absorbent granules comprising an intimate mixture of particles of alumina trihydrate, sodium carbonate or sodium bicarbonate or mixtures thereof and a binder, in which the sodium oxide content Na2O is at least 20% by weight calculated on an ignited (900° C.) base. This material was designated for use at temperatures below 150° C.
A recent patent, U.S. Pat. No. 6,558,641 to ICI, disclosed an attempt to improve the absorbent that was previously disclosed in the above discussed U.S. Pat. No. 5,897,845. This '641 patent taught the use of an alumina combined with both a zinc and an alkali metal component selected from the group consisting of oxides, hydroxides, carbonates, bicarbonates and basic carbonates as well as a binder. It is noted that these two patents disclosed both methods for manufacturing and for the use of the absorbent. The term “absorbent” is used to describe the scavenger which implies that the contaminant uptake while using the material occurs not only by adsorption but also by chemical reaction with the material.
In addition to noting the above prior art, a review of three classes of existing industrial HCl scavengers that are currently being marketed reveals undesirable characteristics of these materials. The alkali or alkaline-earth doped aluminas have an alkali metal content that is between 8 and 14% calculated as the oxide (Na2O for example). The relatively low equilibrium chloride loading of typically 12 to 14% is the biggest problem with these doped aluminas. Intimate mixtures of alumina, carbonate (bicarbonate) and binder are a second class of HCl scavengers. In U.S. Pat. No. 5,897,845 are described adsorbents having a Na2O content that is at least 20 wt-%. This higher level of the oxide indicates that the material has a high potential chloride loading. However, poor stability, tendency of caking and development of pressure drop upon adsorption of moisture, along with difficulties in discharging the spent adsorbent are among the disadvantages of this group. In addition, scavengers of this type cannot be used at higher temperature than 150° C. They have low BET surface area and insufficient porosity. Finally, there are several zinc containing scavengers such as described in the ICI patent. According to U.S. Pat. No. 6,558,641, the HCl scavenger described therein contains an alkali metal component and a binder beside the zinc component. The additional zinc component substantially increases the cost to manufacture the material. This increased cost is a significant disadvantage of this type of HCl scavengers. Also there are some performance issues including indications that the mass-transfer in these materials is slower than in the alumina based adsorbents. This leads to a reduced dynamic capacity. Finally, this group of scavengers may also exhibit reduced mechanical stability at the conditions of use.
In spite of the improvements in industrial scavengers of acid gases such as HCl, there remains a need to achieve further progress in this area. The specific technical problem that remains to be solved is to increase the chloride loading while maintaining sufficient BET surface area, porosity and mechanical stability of the solid scavenger combined with low tendency to produce “green oil”.